Method for d2d communication performed by terminals in wireless communication system, and devices for supporting same

ABSTRACT

Provided are a method for device to device (D2D) communication performed by terminals in a wireless communication system, and devices therefor. The method is characterized by: determining whether or not a first signal for D2D communication with another terminal and a second signal for communication with a base station collide at the same transmission time; and transmitting only the second signal at the transmission time if the first signal and the second signal collide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication and, moreparticularly, to a device to device (D2D) communication method performedby user equipment in a wireless communication system and user equipmentsupporting the same.

2. Related Art

[2] In International Telecommunication Union Radio communication sector(ITU-R), a standardization task for International MobileTelecommunication (IMT)-Advanced, that is, the next-generation mobilecommunication system since the third generation, is in progress.IMT-Advanced sets its goal to support Internet Protocol (IP)-basedmultimedia services at a data transfer rate of 1 Gbps in the stop andslow-speed moving state and of 100 Mbps in the fast-speed moving state.

For example, 3rd Generation Partnership Project (3GPP) is a systemstandard to satisfy the requirements of IMT-Advanced and is preparingfor LTE-Advanced improved from Long Term Evolution (LTE) based onOrthogonal Frequency Division Multiple Access (OFDMA)/SingleCarrier-Frequency Division Multiple Access (SC-FDMA) transmissionschemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a Device-to-Device (D22) technology inwhich devices perform direct communication. In particular, D2D has beenin the spotlight as a communication technology for a public safetynetwork. A commercial communication network is rapidly changing to LTE,but the current public safety network is basically based on the 2Gtechnology in terms of a collision problem with existing communicationstandards and a cost. Such a technology gap and a need for improvedservices are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliabilityand security) than the commercial communication network. In particular,if coverage of cellular communication is not affected or available, thepublic safety network also requires direct communication betweendevices, that is, D2D communication.

D2D communication may have various advantages in that it iscommunication between devices in proximity. For example, D2D UE has ahigh transfer rate and a low delay and may perform data communication.Furthermore, in D2D communication, traffic concentrated on a basestation can be distributed. If D2D UE plays the role of a relay, it mayalso play the role of extending coverage of a base station.

Meanwhile, one of important procedures when D2D communication isperformed is to discover another device in proximity. To this end, D2DUE sends a discovery request signal (it may be hereinafter called adiscovery request signal), and another D2D UE may send a discoveryresponse signal in response to the discovery request signal.Furthermore, in D2D communication, direct communication is performedbetween two types of UE.

Meanwhile, a collision between a D2D signal used in D2D communicationand another signal may be generated in UE attempting to perform D2Dcommunication. If UE sends a D2D signal without affecting transmissionquality of another signal or requirements, the UE may send a D2D signaland another signal at the same time.

However, practically, although a D2D signal and another signal aretransmitted in different frequency bands, they inevitably affect eachother. Accordingly, if a point of time at which a D2D signal istransmitted collides against a point of time at which another signal istransmitted, there is a need for a D2D communication method and devicein which how the collision will be handled is taken into consideration.

SUMMARY OF THE INVENTION

The present invention provides a D2D communication method in a wirelesscommunication system and a device supporting the same.

In an aspect, a device to device (D2D) communication method performed byuser equipment in a wireless communication system is provide. The methodcomprises determining whether a first signal for D2D communication withanother user equipment and a second signal for communication with an eNBcollide with each other at an identical transmission point of time andsending only the second signal at the transmission point of time if thefirst signal and the second signal collide with each other.

The second signal is an uplink signal unicasted to the eNB.

The first signal is a discovery signal for discovering the another userequipment or data transmitted to the another user equipment.

A resource capable of sending the first signal is a first frequencyband, a resource capable of sending the second signal is a secondfrequency band, and the first and the second frequency bands aredifferent frequency bands.

When the user equipment communicates with a first eNB and a second eNB,the user equipment sends only a second signal for communication with thefirst eNB if the second signal for communication with the first eNB anda first signal collide with each other at an identical transmissionpoint of time, and the user equipment sends only a first signal if asecond signal for communication with the second eNB and the first signalcollide with each other at an identical transmission point of time.

The user equipment communicates with the first eNB using a firstfrequency band and communicates with the second eNB using a secondfrequency band, and the first and the second frequency bands aredifferent frequency bands.

If the first signal and the second signal collide with each other, thetransmission of the first signal is abandoned or postponed to at a nexttransmission point of time.

In another aspect, a user equipment performing device to device (D2D)communication in a wireless communication system is provided. The userequipment comprises a Radio Frequency (RF) unit sending and receivingradio signals and a processor operatively connected to the RF unit,wherein the processor determines whether a first signal for D2Dcommunication with another user equipment and a second signal forcommunication with an eNB collide with each other at an identicaltransmission point of time and sends only the second signal at thetransmission point of time if the first signal and the second signalcollide with each other.

In accordance with the present invention, a specific signal may betransmitted based on priority when a collision is generated by takinginto consideration whether a first signal for D2D communication withanother UE and a second signal for communication with an eNB collidewith each other. If priority is given to the second signal, theinfluence of the signal used in communication with the eNB due to thesignal for D2D communication can be prevented. If priority is given tothe first signal, more reliable D2D communication is possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates substrates which may be owned by UE in the RRC_IDLEstate and a substrate transition process.

FIG. 9 shows the structure of a radio frame in 3GPP LTE.

FIG. 10 shows an example of a resource grid for one downlink slot.

FIG. 11 shows the structure of an uplink subframe.

FIG. 12 is a flowchart illustrating a contention-based random accessprocess.

FIG. 13 shows a basic structure for ProSe.

FIG. 14 shows examples in which types of UE performing ProSe directcommunication and cell coverage are deployed.

FIG. 15 shows a user plane protocol stack for ProSe directcommunication.

FIG. 16 shows a PC 5 interface for D2D direct discovery.

FIG. 17 is an embodiment of a ProSe discovery process.

FIG. 18 is another embodiment of a ProSe discovery process.

FIG. 19 shows a D2D communication method of UE according to anembodiment of the present invention.

FIG. 20 illustrates the state of UE to which the present invention maybe applied.

FIG. 21 is a block diagram showing UE in which the embodiments of thepresent invention are implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a process of defining the characteristicsof a wireless protocol layer and channels in order to provide specificservice and configuring each detailed parameter and operating method. AnRB can be divided into two types of a Signaling RB (SRB) and a Data RB(DRB). The SRB is used as a passage through which an RRC message istransmitted on the control plane, and the DRB is used as a passagethrough which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L 1/L2 control channel. A Transmission TimeInterval (TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a process of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.

Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include the limited number of parameters which are the mostessential and are most frequently transmitted in order to obtain otherinformation from a cell. UE first discovers an MIB after downlinksynchronization. The MIB may include information, such as a downlinkchannel bandwidth, a PHICH configuration, an SFN supportingsynchronization and operating as a timing reference, and an eNBtransmission antenna configuration. The MIB may be broadcasted on a BCH.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. Other SIBs otherthan the SIB1 are included in a system information message andtransmitted. The mapping of the SIBs to the system information messagemay be flexibly configured by a scheduling information list parameterincluded in the SIB1. In this case, each SIB is included in a singlesystem information message. Only SIBs having the same schedulingrequired value (e.g. period) may be mapped to the same systeminformation message. Furthermore, SystemInformationBlockType2 (SIB2) isalways mapped to a system information message corresponding to the firstentry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same period. The SIB1 and all of the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in the E-UTRAN, the SIB1 may bechannel-dedicated signaling including a parameter set to have the samevalue as an existing set value. In this case, the SIB1 may be includedin an RRC connection re-establishment message and transmitted.

The SIB1 includes information related to UE cell access and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers, Tracking Area Code (TAC), and cell ID of anetwork, a cell barring state indicative of whether a cell is a cell onwhich UE can camp, a required minimum reception level within a cellwhich is used as a cell reselection reference, and the transmission timeand period of other SIBs.

The SIB2 may include radio resource configuration information common toall types of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and fordetecting a change of system information to only a PCell. In an SCell,when the corresponding SCell is added, the E-UTRAN may provide all typesof system information related to an RRC connection state operationthrough dedicated signaling. When system information related to aconfigured SCell is changed, the E-UTRAN may release a considered SCelland add the considered SCell later. This may be performed along with asingle RRC connection re-establishment message. The E-UTRAN may set avalue broadcast within a considered SCell and other parameter valuethrough dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation. Such system information is called required systeminformation. The required system information may be defined as follows.

If UE is in the RRCIDLE state: the UE needs to have the valid version ofthe MIB and the SIB1 in addition to the SIB2 to SIB8. This may complywith the support of a considered RAT.

If UE is in the RRC connection state: the UE needs to have the validversion of the MIB, SIB1, and SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after being obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection process, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel.Thereafter, if the UE has only to search for a suitable cell thatsatisfies a cell selection criterion, the UE selects the correspondingcell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RAEand frequency characteristics of the cell.

Intra-frequency cell reselection: UE reselects a cell having the samecenter frequency as that of RAT, such as a cell on which the UE campson.

Inter-frequency cell reselection: UE reselects a cell having a differentcenter frequency from that of RAT, such as a cell on which the UE campson

Inter-RAT cell reselection: UE reselects a cell that uses RAT differentfrom RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.

Rs=Qmeas,s+Qhyst,Rn=Qmeas,s−Qoffset  [Equation 1]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

A Radio Link Failure (RLF) is described below.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

A case where UE determines that there is a serious problem in thequality of a downlink communication link (a case where the quality of aPCell is determined to be low while performing RLM) based on the radioquality measured results of the PHY layer of the UE

A case where uplink transmission is problematic because a random accessprocedure continues to fail in the MAC sublayer.

A case where uplink transmission is problematic because uplink datatransmission continues to fail in the RLC sublayer.

A case where handover is determined to have failed.

A case where a message received by UE does not pass through an integritycheck.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this process, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates substrates which may be owned by UE in the RRC_IDLEstate and a substrate transition process.

Referring to FIG. 8, UE performs an initial cell selection process(S801). The initial cell selection process may be performed when thereis no cell information stored with respect to a PLMN or if a suitablecell is not discovered.

If a suitable cell is unable to be discovered in the initial cellselection process, the UE transits to any cell selection state (S802).The any cell selection state is the state in which the UE has not campedon a suitable cell and an acceptable cell and is the state in which theUE attempts to discover an acceptable cell of a specific PLMN on whichthe UE may camp. If the UE has not discovered any cell on which it maycamp, the UE continues to stay in the any cell selection state until itdiscovers an acceptable cell.

If a suitable cell is discovered in the initial cell selection process,the UE transits to a normal camp state (S803). The normal camp staterefers to the state in which the UE has camped on the suitable cell. Inthis state, the UE may select and monitor a paging channel based oninformation provided through system information and may perform anevaluation process for cell reselection.

If a cell reselection evaluation process (S804) is caused in the normalcamp state (S803), the UE performs a cell reselection evaluation process(S804). If a suitable cell is discovered in the cell reselectionevaluation process (S804), the UE transits to the normal camp state(S803) again.

If an acceptable cell is discovered in the any cell selection state(S802), the UE transmits to any cell camp state (S805). The any cellcamp state is the state in which the UE has camped on the acceptablecell.

In the any cell camp state (S805), the UE may select and monitor apaging channel based on information provided through system informationand may perform the evaluation process (S806) for cell reselection. Ifan acceptable cell is not discovered in the evaluation process (S806)for cell reselection, the UE transits to the any cell selection state(S802).

As described above, the RRC_IDLE state may have three types ofsubstrates, such as 1) the normal camp state, 2) the any cell campstate, and 3) the any cell selection state.

The structure of a radio frame and an uplink physical channel in 3GPPLTE/LTE-A are described below.

FIG. 9 shows the structure of a radio frame in 3GPP LTE. For thestructure, reference may be made to Paragraph 4 of 3GPP TS 36.211 V8.7.0(2009-05) “Evolved Universal Terrestrial Radio Access (the E-UTRA);Physical Channels and Modulation (Release 8).”

The radio frame includes 10 subframes assigned indices of 0˜9. Onesubframe includes 2 contiguous slots. The time taken for one subframe tobe transmitted is called a Transmission Time Interval (TTI). Forexample, the length of one subframe may be 1 ms, and the length of oneslot may be 0.5 ms. Each subframe may include two slots in a timedomain. A downlink subframe may include 2 downlink slots, and an uplinksubframe may include 2 uplink slots.

FIG. 10 shows an example of a resource grid for one downlink slot.

Referring to FIG. 10, the downlink slot includes a plurality of OFDMsymbols in a time domain and includes N_(RB) Resource Blocks (RBs) in afrequency domain. The RB is a resource allocation unit and includes oneslot in the time domain and a plurality of contiguous subcarriers in thefrequency domain. The number of RBs N_(RB) included in the downlink slotdepends on a downlink transmission bandwidth configured in a cell. Forexample, in an LTE system, N_(RB) may be any one of 6 to 110. Thestructure of an uplink slot may be the same as that of the downlinkslot.

Each element on the resource grid is called a Resource Element (RE). Theresource element on the resource grid may be identified by an index pair(k,l) within a slot. In this case, k (k=0, . . . , N_(RB)×12−1) is theindices of subcarriers in the frequency domain, and 1 (1=0, . . . , 6)is the indices of OFDM symbols in the time domain.

In FIG. 10, one resource block has been illustrated as including 7 OFDMsymbols in the time domain and 12 subcarriers in the frequency domainand as including 7×12 resource elements, but the number of OFDM symbolsand the number of subcarriers within the resource block are not limitedthereto. In a normal CP, 1 slot may include 7 OFDM symbols. In anextended CP, 1 slot may include 6 OFDM symbols. The number of OFDMsymbols and the number of subcarriers may be changed in various waysdepending on the length of a CP, the frequency spacing, etc. In one OFDMsymbol, one of 128, 256, 512, 1024, 1536, and 2048 may be selected andused as the number of subcarriers.

FIG. 11 shows the structure of an uplink subframe.

Referring to FIG. 11, the uplink subframe may be divided into a controlregion and a data region in a frequency domain. Physical Uplink ControlChannels (PUCCHs) on which uplink control information is transmitted areallocated to the control region. Physical Uplink Shared Channels(PUSCHs) on which data is transmitted (control information may also betransmitted according to circumstances) are allocated to the dataregion. UE may send a PUCCH and a PUSCH at the same time or may send anyone of a PUCCH and a PUSCH depending on a configuration.

A PUCCH for a piece of UE is allocated as an RB pair in a subframe. RBsbelonging to an RB pair occupy different subcarriers in a first slot anda second slot. A frequency occupied by an RB belonging to an RB pairallocated to a PUCCH is changed based on a slot boundary. This is saidthat the RB pair allocated to the PUCCH has been frequency-hopped in theslot boundary. A frequency diversity gain can be obtained by sendinguplink control information through different subcarriers over time.

Hybrid Automatic Repeat reQuest (HARQ) AcknowledgementACK)/Not-acknowledgement (NACK), Channel Status Information (CSI)indicative of a downlink channel status, for example, a Channel QualityIndicator (CQI) may be transmitted on a PUCCH. Data and/or controlinformation, such as ACK/NACK and CSI, may be transmitted on a PUSCH.

A random access process is described below.

UE may perform a random access process on an eNB in the following cases.

(1) When the UE performs initial access without RRC connection with aneNB, (2) When the UE first accesses a target cell in a handover process,(3) When a request is made by a command from the eNB, (4) When uplinkdata is generated if uplink time alignment has not been matched oruplink radio resources have not been allocated, (5) in the case of arecovery process when a radio link failure or handover failure isgenerated.

A random access process includes two types of a contention-based randomaccess procedure and a non-contention-based random access procedure. Inthis case, the contention means that two types of UE attempt randomaccess processes using the same random access preamble through the sameresource.

FIG. 12 is a flowchart illustrating a contention-based random accessprocess.

Referring to FIG. 12, at step S110, UE selects a specific one randomaccess preamble from a random access preamble set and sends the selectedrandom access preamble to an eNB through a PRACH resource. Informationabout the configuration of the random access preamble set may beobtained from some of system information or the eNB through a handovercommand message.

At step S120, the UE attempts to receive its own random access responsewithin a random access response reception window. The random accessresponse reception window may be indicated through some of the systeminformation or the handover command message and refers to a window inwhich a random access response is monitored. More specifically, therandom access response is transmitted in a Medium Access Control (MAC)Protocol Data Unit (PDU) format, and the MAC PDU is transmitted to aPDSCH, that is, a physical channel. The received information of thePDSCH is obtained through a PDCCH, that is, a control channel. The PDCCHcarries information about UE that will receive the PDSCH, radioresources allocation information of the PDSCH, the transport format ofthe PDSCH and so on. First, when UE succeeds in receiving a PDCCH bymonitoring the PDCCH within a subframe belonging to a random accessresponse reception window, the UE receives a random access response on aPDSCH indicated by the PDCCH.

The random access response includes a Time Alignment (TA) value for theuplink synchronization of the UE, uplink radio resource allocationinformation, and temporary identifiers of the UE, such as a RandomAccess Preamble IDentifier (RAPID) and a temporary Cell-Radio NetworkTemporary Identity (C-RNTI) for identifying types of UE performingrandom access. The random access preamble identifier is for identifyinga received random access preamble.

At step S130, the UE applies the time alignment value and sends ascheduled message, including the random access identifier, to the eNBusing the uplink radio resources allocation information.

The random access identifier is used for the eNB to identify UEperforming a random access process. The random access identifier may beobtained using two methods. In the first method, if UE has a valid cellidentifier (e.g., a C-RNTI) already allocated by a corresponding cellprior to a random access process, the UE uses the cell identifier as arandom access identifier. In the second method, if UE does not have avalid cell identifier allocated thereto prior to a random accessprocess, the UE uses an SAE Temporary Mobile Station Identifier (S-TMSI)or a higher layer identifier as a random access identifier. The UEstarts a contention resolution timer by sending a scheduled message.

At step S140, after receiving the scheduled message, the eNB sends acontention resolution message, including the random access identifier,to the UE. In a contention-based random access process, what UE is madeaware of whether a contention has failed or succeeded is called acontention resolution.

Proximity services (ProSe) is described.

The ProSe is a concept which may include D2D communication. Hereinafter,the ProSe may be interchangeably used along with D2D.

ProSe direct communication refers to communication performed between twoor more types of adjacent UE. The types of UE may perform communicationusing the protocol of the user plane. ProSe-enabled UE means UEsupporting a procedure related to the requirements of ProSe. Unlessotherwise described, the ProSe-enabled UE includes both public safety UEand non-public safety UE. The public safety UE is UE supporting both afunction specified for public safety and a ProSe process. The non-publicsafety UE is UE supporting a ProSe process, but not supporting afunction specified for public safety.

ProSe direct discovery is a process for discovering anotherProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only thecapabilities of the two types of ProSe-enabled UE are used. EPC-levelProSe discovery means a process for determining, by an EPC, whether thetwo types of ProSe-enabled UE are in proximity and notifying the twotypes of ProSe-enabled UE of the proximity.

FIG. 13 shows a basic structure for ProSe.

Referring to FIG. 13, the basic structure for ProSe includes an E-UTRAN,an EPC, a plurality of types of UE including a ProSe applicationprogram, a ProSe application server (a ProSe APP server), and a ProSefunction.

The EPC represents an E-UTRAN core network configuration. The EPC mayinclude an MME, an S-GW, a P-GW, a policy and charging rules function(PCRF), a home subscriber server (HSS) and so on.

The ProSe APP server is a user of a ProSe capability for producing anapplication function. The ProSe APP server may communicate with anapplication program within UE. The application program within UE may usea ProSe capability for producing an application function.

The ProSe function may include at least one of the followings, but isnot necessarily limited thereto.

-   -   Interworking via a reference point toward the 3rd party        applications    -   Authorization and configuration of UE for discovery and direct        communication    -   Enable the functionality of EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of the ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of the EPC,        e.g., offline charging)

A reference point and a reference interface in the basic structure forProSe are described below.

PC1: a reference point between the ProSe application program within theUE and the ProSe application program within the ProSe APP server. Thisis used to define signaling requirements in an application dimension.

PC2: a reference point between the ProSe APP server and the ProSefunction. This is used to define an interaction between the ProSe APPserver and the ProSe function. The update of application data in theProSe database of the ProSe function may be an example of theinteraction.

PC3: a reference point between the UE and the ProSe function. This isused to define an interaction between the UE and the ProSe function. Aconfiguration for ProSe discovery and communication may be an example ofthe interaction.

PC4: a reference point between the EPC and the ProSe function. This isused to define an interaction between the EPC and the ProSe function.The interaction may illustrate the time when a path for 1:1communication between types of UE is set up or the time when ProSeservice for real-time session management or mobility management isauthenticated.

PC5: a reference point used for using control/user plane for discoveryand communication, relay, and 1:1 communication between types of UE.

PC6: a reference point for using a function, such as ProSe discovery,between users belonging to different PLMNs.

SGi: this may be used to exchange application data and types ofapplication dimension control information.

<ProSe Direct Communication>

ProSe direct communication is communication mode in which two types ofpublic safety UE can perform direct communication through a PC 5interface. Such communication mode may be supported when UE is suppliedwith services within coverage of an E-UTRAN or when UE deviates fromcoverage of an E-UTRAN.

FIG. 14 shows examples in which types of UE performing ProSe directcommunication and cell coverage are deployed.

Referring to FIG. 14 (a), types of UE A and B may be placed outside cellcoverage. Referring to FIG. 14 (b), UE A may be placed within cellcoverage, and UE B may be placed outside cell coverage. Referring toFIG. 14 (c), types of UE A and B may be placed within single cellcoverage. Referring to FIG. 14 (d), UE A may be placed within coverageof a first cell, and UE B may be placed within coverage of a secondcell.

ProSe direct communication may be performed between types of UE placedat various positions as in FIG. 14.

Meanwhile, the following IDs may be used in ProSe direct communication.

A source layer-2 ID: this ID identifies the sender of a packet in the PC5 interface.

A destination layer-2 ID: this ID identifies the target of a packet inthe PC 5 interface.

An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5interface.

FIG. 15 shows a user plane protocol stack for ProSe directcommunication.

Referring to FIG. 15, the PC 5 interface includes a PDCH, RLC, MAC, andPHY layers.

In ProSe direct communication, HARQ feedback may not be present. An MACheader may include a source layer-2 ID and a destination layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>

ProSe-enabled UE may use the following two types of mode for resourceassignment for ProSe direct communication.

1. Mode 1

Mode 1 is mode in which resources for ProSe direct communication arescheduled by an eNB. UE needs to be in the RRC_CONNECTED state in orderto send data in accordance with mode 1. The UE requests a transmissionresource from an eNB. The eNB performs scheduling assignment andschedules resources for sending data. The UE may send a schedulingrequest to the eNB and send a ProSe Buffer Status Report (BSR). The eNBhas data to be subjected to ProSe direct communication by the UE basedon the ProSe BSR and determines that a resource for transmission isrequired.

2. Mode 2

Mode 2 is mode in which UE directly selects a resource. UE directlyselects a resource for ProSe direct communication in a resource pool.The resource pool may be configured by a network or may have beenpreviously determined.

Meanwhile, if UE has a serving cell, that is, if the UE is in theRRC_CONNECTED state with an eNB or is placed in a specific cell in theRRC_IDLE state, the UE is considered to be placed within coverage of theeNB.

If UE is placed outside coverage, only mode 2 may be applied. If the UEis placed within the coverage, the UE may use mode 1 or mode 2 dependingon the configuration of an eNB.

If another exception condition is not present, only when an eNB performsa configuration, UE may change mode from mode 1 to mode 2 or from mode 2to mode 1.

<ProSe Direct Discovery>

ProSe direct discovery refers to a procedure that is used forProSe-enabled UE to discover another ProSe-enabled UE in proximity andis also called D2D direct discovery. In this case, E-UTRA radio signalsthrough the PC 5 interface may be used. Information used in ProSe directdiscovery is hereinafter called discovery information.

FIG. 16 shows a PC 5 interface for D2D direct discovery.

Referring to FIG. 16, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer, that is, a higher layer. The higherlayer (the ProSe Protocol) handles the permission of the announcementand monitoring of discovery information. The contents of the discoveryinformation are transparent to an access stratum (AS). The ProSeProtocol transfers only valid discovery information to the AS forannouncement.

The MAC layer receives discovery information from the higher layer (theProSe Protocol). An IP layer is not used to send discovery information.The MAC layer determines a resource used to announce discoveryinformation received from the higher layer. The MAC layer produces anMAC protocol data unit (PDU) for carrying discovery information andsends the MAC PDU to the physical layer. An MAC header is not added.

In order to announce discovery information, there are two types ofresource assignment.

1. Type 1

The type 1 is a method for assigning a resource for announcing discoveryinformation in a UE-not-specific manner. An eNB provides a resource poolconfiguration for discovery information announcement to types of UE. Theconfiguration may be signaled through the SIB.

UE autonomously selects a resource from an indicated resource pool andannounces discovery information using the selected resource. The UE mayannounce the discovery information through a randomly selected resourceduring each discovery period.

2. Type 2

The type 2 is a method for assigning a resource for announcing discoveryinformation in a UE-specific manner. UE in the RRC_ONNECTED state mayrequest a resource for discovery signal announcement from an eNB throughan RRC signal. The eNB may announce a resource for discovery signalannouncement through an RRC signal. A resource for discovery signalmonitoring may be assigned within a resource pool configured for typesof UE.

An eNB 1) may announce a type 1 resource pool for discovery signalannouncement to UE in the RRC_IDLE state through the SIB. Types of UEwhose ProSe direct discovery has been permitted use the type 1 resourcepool for discovery information announcement in the RRC_IDLE state.Alternatively, the eNB 2) announces that the eNB supports ProSe directdiscovery through the SIB, but may not provide a resource for discoveryinformation announcement. In this case, UE needs to enter theRRC_CONNECTED state for discovery information announcement.

An eNB may configure that UE has to use a type 1 resource pool fordiscovery information announcement or has to use a type 2 resourcethrough an RRC signal in relation to UE in the RRC_CONNECTED state.

FIG. 17 is an embodiment of a ProSe discovery process.

Referring to FIG. 17, it is assumed that UE A and UE B haveProSe-enabled application programs managed therein and have beenconfigured to have a ‘friend’ relation between them in the applicationprograms, that is, a relationship in which D2D communication may bepermitted between them. Hereinafter, the UE B may be represented as a‘friend’ of the UE A. The application program may be, for example, asocial networking program. ‘3GPP Layers’ correspond to the functions ofan application program for using ProSe discovery service, which havebeen defined by 3GPP.

Direct discovery between the types of UE A and B may experience thefollowing process.

1. First, the UE A performs regular application layer communication withthe APP server. The communication is based on an Application ProgramInterface (API).

2. The ProSe-enabled application program of the UE A receives a list ofapplication layer IDs having a ‘friend’ relation. In general, theapplication layer ID may have a network access ID form. For example, theapplication layer ID of the UE A may have a form, such as“adam@example.com.”

3. The UE A requests private expressions code for the user of the UE Aand private representation code for a friend of the user.

4. The 3GPP layers send a representation code request to the ProSeserver.

5. The ProSe server maps the application layer IDs, provided by anoperator or a third party APP server, to the private representationcode. For example, an application layer ID, such as adam@example.com,may be mapped to private representation code, such as“GTER543$#2FSJ67DFSF.” Such mapping may be performed based on parameters(e.g., a mapping algorithm, a key value and so on) received from the APPserver of a network.

6. The ProSe server sends the types of derived representation code tothe 3GPP layers. The 3GPP layers announce the successful reception ofthe types of representation code for the requested application layer IDto the ProSe-enabled application program. Furthermore, the 3GPP layersgenerate a mapping table between the application layer ID and the typesof representation code.

7. The ProSe-enabled application program requests the 3GPP layers tostart a discovery procedure. That is, the ProSe-enabled applicationprogram requests the 3GPP layers to start discovery when one of provided‘friends’ is placed in proximity to the UE A and direct communication ispossible. The 3GPP layers announces the private representation code(i.e., in the above example, “GTER543$#2FSJ67DFSF”, that is, the privaterepresentation code of adam@example.com) of the UE A. This ishereinafter called ‘announcement’. Mapping between the application layerID of the corresponding application program and the privaterepresentation code may be known to only ‘friends’ which have previouslyreceived such a mapping relation, and the ‘friends’ may perform suchmapping.

8. It is assumed that the UE B operates the same ProSe-enabledapplication program as the UE A and has executed the aforementioned 3 to6 steps. The 3GPP layers placed in the UE B may execute ProSe discovery.

9. When the UE B receives the aforementioned ‘announce’ from the UE A,the UE B determines whether the private representation code included inthe ‘announce’ is known to the UE B and whether the privaterepresentation code is mapped to the application layer ID. As describedthe 8 step, since the UE B has also executed the 3 to 6 steps, it isaware of the private representation code, mapping between the privaterepresentation code and the application layer ID, and correspondingapplication program of the UE A. Accordingly, the UE B may discover theUE A from the ‘announce’ of the UE A. The 3GPP layers announce thatadam@example.com has been discovered to the ProSe-enabled applicationprogram within the UE B.

In FIG. 17, the discovery procedure has been described by taking intoconsideration all of the types of UE A and B, the ProSe server, the APPserver and so on. From the viewpoint of the operation between the typesof UE A and B, the LIE A sends (this process may be called announcement)a signal called announcement, and the UE B receives the announce anddiscovers the UE A. That is, from the aspect that an operation thatbelongs to operations performed by types of UE and that is directlyrelated to another UE is only step, the discovery process of FIG. 17 mayalso be called a single step discovery procedure.

FIG. 18 is another embodiment of a ProSe discovery process.

In FIG. 18, types of UE 1 to 4 are assumed to types of UE included inspecific group communication system enablers (GCSE) group. It is assumedthat the UE 1 is a discoverer and the types of UE 2, 3, and 4 arediscovered. UE 5 is UE not related to the discovery process.

The UE 1 and the UE 2-4 may perform a next operation in the discoveryprocess.

First, the UE 1 broadcasts a target discovery request message (may behereinafter abbreviated as a discovery request message or MD in order todiscover whether specific UE included in the GCSE group is in proximity.The target discovery request message may include the unique applicationprogram group ID or layer-2 group ID of the specific GCSE group.Furthermore, the target discovery request message may include the uniqueID, that is, application program private ID of the UE 1. The targetdiscovery request message may be received by the types of UE 2, 3, 4,and 5.

The UE 5 sends no response message. In contrast, the types of UE 2, 3,and 4 included in the GCSE group send a target discovery responsemessage (may be hereinafter abbreviated as a discovery response messageor M2) as a response to the target discovery request message. The targetdiscovery response message may include the unique application programprivate ID of UE sending the message.

An operation between types of UE in the ProSe discovery processdescribed with reference to FIG. 18 is described below. The discoverer(the UE 1) sends a target discovery request message and receives atarget discovery response message, that is, a response to the targetdiscovery request message. Furthermore, when the discoveree (e.g., theUE 2) receives the target discovery request message, it sends a targetdiscovery response message, that is, a response to the target discoveryrequest message. Accordingly, each of the types of UE performs theoperation of the 2 step. In this aspect, the ProSe discovery process ofFIG. 14 may be called a 2-step discovery procedure.

In addition to the discovery procedure described in FIG. 18, if the UE 1(the discoverer) sends a discovery conform message (may be hereinafterabbreviated as an M3), that is, a response to the target discoveryresponse message, this may be called a 3-step discovery procedure.

The present invention is described below. A collision between a D2Dsignal and another signal may be generated in UE attempting to performD2D communication. If UE sends a D2D signal without affectingtransmission quality of another signal or requirements, the UE may sendthe D2D signal and another signal at the same time.

However, practically, although a D2D signal and another signal (e.g., asignal used in communication with an eNB in a cellular system) aretransmitted in different frequency bands, they inevitably affect eachother. Furthermore, a point of time at which UE attempts to performcellular transmission in uplink may collide against a point of time atwhich the UE attempts to send a D2D signal in the same frequency band inuplink. Accordingly, if a point of time at which a D2D signal istransmitted collides against a point of time at which another signal istransmitted, there is a need for a D2D communication method in which howthe collision will be handled is taken into consideration.

FIG. 19 shows a D2D communication method of UE according to anembodiment of the present invention.

Referring to FIG. 19, the UE determines whether a first signal for D2Dcommunication with another UE collides against a second signal forcommunication with an eNB at the same transmission point of time (S111).

If the first signal and the second signal collide with each other, theUE sends only the second signal at the transmission point of time(S112). That is, the second signal has higher priority than the firstsignal in transmission. In this case, the transmission of the firstsignal may be abandoned or postponed to a next transmission point oftime.

In this case, the first signal may be a discovery signal that isnecessary to be transmitted by the UE in order to discover another UE onwhich D2D communication is to be performed, data transmitted to anotherUE through ProSe direct communication and so on.

The second signal may be an uplink signal unicasted from the UE to theeNB. Unicast means that data is transmitted to one destination that isuniquely identified unlike broadcast in which the same data istransmitted to all of transmittable destinations.

The second signal may be, for example, an uplink signal transmitted bythe UE through a PUSCH, an uplink signal transmitted through a PUCCH, ascheduling request, channel status information represented by a ChannelQuality Indicator (CQI), acknowledgement/not-acknowledgement (ACK/NACK)indicative of the acknowledgement of the reception of data in an HARQprocess, a preamble in a random access process and so on.

The UE is unable to send the first signal and a preamble in a randomaccess process at the same time. If a point of time at which the firstsignal is transmitted overlaps a point of time at which the preamble istransmitted, the UE puts priority on the transmission of the preamble.

If a random access process is caused, the UE may suspend thetransmission of the first signal. If the transmission of the firstsignal is suspended, the transmission of the first signal may resumewhen the caused random access process is successfully terminated or anext event is generated.

For example, the event may be an event that is terminated because 1) ascheduled message is successfully received by the eNB, 2) the eNBsuccessfully sends a contention resolution message, or 3) a randomaccess process is repeated or reset by a maximum permissible number inthe random access process.

Meanwhile, a resource in which the first signal may be transmitted maybe a first frequency band, and a resource in which the second signal maybe transmitted may be a second frequency band. In this case, the firstand the second frequency bands may be different frequency bands.

FIG. 20 illustrates the state of UE to which the present invention maybe applied.

Referring to FIG. 20, a first cell 201 in which a first eNB providesservice and a second cell 202 in which a second eNB provides service maybe deployed near types of UE A, B, C, and D. It is assumed that thefirst eNB provides the service through a first frequency band and thesecond eNB provides the service through a second frequency band. Thefirst and the second frequency band may be the same or may be different.In this case, it is assumed that the first and the second frequency bandare different bands, for convenience sake. In FIG. 20, the first and thesecond frequency bands have been illustrated as including an uplinkcarrier (UL carrier) and downlink carrier (DL carrier) of an FDD method,respectively. However, the first and the second frequency bands may beformed of one carrier as in a TDD method.

The UE A is placed in the first cell 201, and the UE B is placed in thefirst cell 201 and the second cell 202 so as to overlap the first andthe second cells. The UE C is placed in the second cell 202, and the UED is placed outside the first and the second cells 201 and 202. It isassumed that the UE placed within the cell is connected to a cell (morespecifically, an eNB providing service to the cell).

In this case, the UE B may be said to be the state in which it has beenconnected to the two cells. Such UE B may put priority on the firstsignal and the second signal using a different method for each cell.

For example, if the second signal (a PUSCH, a PUCCH, a SchedulingRequest (SR), a CQI and so on: the SR is a signal through which UErequests scheduling from an eNB for uplink transmission and the CQI ischannel status information indicative of quality of a downlink channel)and the first signal (a signal for D2D communication) for communicationwith the first eNB collide with each other at the same transmissionpoint of time, the UE B may send only the second signal forcommunication with the first eNB. That is, the UE B may put priority onthe second signal as described with reference to FIG. 19 with respect tothe first cell.

In contrast, if the second signal (a PUSCH, a PUCCH, an SR, a CQI and soon) for communication with the second eNB and the first signal (a signalfor D2D communication) collide with each other at the same transmissionpoint of time, the UE B may put priority on the first signal.

That is, in the present invention, a specific signal may be transmittedbased on priority when a collision is generated by taking intoconsideration whether the first signal and the second signal collidewith each other. If priority is put on the second signal, the influenceof a signal used for communication with the eNB due to a signal for D2Dcommunication can be prevented. If priority is put on the first signal,more reliable D2D communication may be possible. A network may configurepriority if necessary or according to circumstances. The network mayconfigure priority by providing UE with priority through systeminformation or configuration information for each UE.

Meanwhile, the UE may control priority depending on the type of trafficcarried on the first signal to be transmitted through D2D communication.For example, traffic carried on the first signal may include informationthat is essential for D2D communication and has a specific format andinformation different for each D2D communication, such as data to betransmitted by the UE. In this case, the information that is essentialfor D2D communication and has a specific format may have higher prioritythan the data. If a point of time at which the first signal istransmitted and a point of time at which the second signal istransmitted collide with each other temporally, the UE may put priorityhigher than the priority of the second signal on a specific type oftraffic of the traffic carried on the first signal. The UE may puthigher priority on the second signal used for communication with the eNBwith respect to other D2D traffic. The specific traffic may be a traffictype instructed by a network or may be a traffic type predetermined forthe UE in relation to the operation.

Meanwhile, the UE may put priority on the second signal in which aspecific traffic type is transmitted. If a point of time at which thefirst signal is transmitted and a point of time at which the secondsignal is transmitted collide with each other temporally, the UE putspriority on a specific type of traffic of the traffic carried on thesecond signal. The specific traffic may be a traffic type instructed bya network or may be a traffic type predetermined for the UE in relationto the operation. For example, the UE may put the highest priority onthe second signal corresponding to control information transferredthrough a control channel.

That is, the first and the second signals may include a variety of typesof traffic, respectively. If priority is determined within the types oftraffic of the first signal, priority is determined within the types oftraffic of the second signal. Furthermore, priority may be determinedwith respect to all of the types of the first signal and the types ofthe second signal. A network may provide the UE with such prioritythrough system information or configuration information for each UE.

Meanwhile, the first cell may be a cell included in a Master Cell Group(MCG) providing RRC connection to the UE B. The second cell may be aSecondary Cell Group (SCG) not providing RRC connection to the UE B.

Alternatively, the first cell may be a serving cell for the UE B, andthe second cell may be an adjacent cell of the UE B.

FIG. 21 is a block diagram showing UE in which the embodiments of thepresent invention are implemented.

Referring to FIG. 21, UE 1100 includes a processor 1110, memory 1120,and a Radio Frequency (RF) unit 1130. The processor 1110 implements theproposed functions, processes and/or methods. For example, the processor1110 may determine whether a first signal for D2D communication withanother UE and a second signal for communication with an eNB collidewith each other at the same transmission point of time and may send onlythe second signal at the transmission point of time if the first signaland the second signal collide with each other. The processor 1110 mayput different priority on the first signal and the second signal foreach cell.

The RF unit 1130 is connected to the processor 1110 and sends andreceives radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A device to device (D2D) communication methodperformed by user equipment in a wireless communication system, themethod comprising: determining whether a first signal for D2Dcommunication with another user equipment and a second signal forcommunication with an eNB collide with each other at an identicaltransmission point of time; and sending only the second signal at thetransmission point of time if the first signal and the second signalcollide with each other.
 2. The method of claim 1, wherein the secondsignal is an uplink signal unicasted to the eNB.
 3. The method of claim1, wherein the first signal is a discovery signal for discovering theanother user equipment or data transmitted to the another userequipment.
 4. The method of claim 1, wherein: a resource capable ofsending the first signal is a first frequency band, a resource capableof sending the second signal is a second frequency band, and the firstand the second frequency bands are different frequency bands.
 5. Themethod of claim 1, wherein when the user equipment communicates with afirst eNB and a second eNB, the user equipment sends only a secondsignal for communication with the first eNB if the second signal forcommunication with the first eNB and a first signal collide with eachother at an identical transmission point of time, and the user equipmentsends only a first signal if a second signal for communication with thesecond eNB and the first signal collide with each other at an identicaltransmission point of time.
 6. The method of claim 5, wherein: the userequipment communicates with the first eNB using a first frequency bandand communicates with the second eNB using a second frequency band, andthe first and the second frequency bands are different frequency bands.7. The method of claim 1, wherein if the first signal and the secondsignal collide with each other, the transmission of the first signal isabandoned or postponed to at a next transmission point of time.
 8. Userequipment performing device to device (D2D) communication in a wirelesscommunication system, the user equipment comprising: a Radio Frequency(RF) unit sending and receiving radio signals; and a processoroperatively connected to the RF unit, wherein the processor determineswhether a first signal for D2D communication with another user equipmentand a second signal for communication with an eNB collide with eachother at an identical transmission point of time and sends only thesecond signal at the transmission point of time if the first signal andthe second signal collide with each other.
 9. The user equipment ofclaim 8, wherein the second signal is an uplink signal unicasted to theeNB.
 10. The user equipment of claim 8, wherein the first signal is adiscovery signal for discovering the another user equipment or datatransmitted to the another user equipment.
 11. The user equipment ofclaim 8, wherein: a resource capable of sending the first signal is afirst frequency band, a resource capable of sending the second signal isa second frequency band, and the first and the second frequency bandsare different frequency bands.
 12. The user equipment of claim 8,wherein when the user equipment communicates with a first eNB and asecond eNB, the user equipment sends only a second signal forcommunication with the first eNB if the second signal for communicationwith the first eNB and a first signal collide with each other at anidentical transmission point of time, and the user equipment sends onlya first signal if a second signal for communication with the second eNBand the first signal collide with each other at an identicaltransmission point of time.
 13. The user equipment of claim 12, wherein:the user equipment communicates with the first eNB using a firstfrequency band and communicates with the second eNB using a secondfrequency band, and the first and the second frequency bands aredifferent frequency bands.
 14. The user equipment of claim 8, wherein ifthe first signal and the second signal collide with each other, thetransmission of the first signal is abandoned or postponed to at a nexttransmission point of time.